The locomotives were essentially modelled on the 1927 rebuild of Locomotive 338 of Class 333.[1]: 113–115, 269–270 The 5 feet 8+1⁄2 inches (1,740 mm) driving wheels and less than 16 long ton maximum axle load were suitable for go anywhere and secondary passenger and mixed duties as opposed to the 6 feet 7 inches (2,010 mm) driving wheels of classes 301 through 321 which were more suited to express passenger work.
Service
In practice all members of these classed were treated interchangeably for operational purposes. A 16 long tons (16 t) maximum axle load allowed the class to be used almost anywhere. The were assessed in 1948 as quite powerful and fast and especially useful for passenger specials.[1]: 113–115, 269–270